This invention relates to analyzers of the double beam, optical null type such as spectrophotometers utilizing beam intensity balancing, and in particular, to spectrophotometers having recording chart drive systems.
In analyzers of this type, radiation from a source is switched along a reference beam path and a sample beam path, the two beams being modulated and recombined at a thermocouple or other electrical signal generator. A sample to be analyzed is placed in one path and an error signal is used to drive a servo which varies the intensity of the beam in the reference path to achieve a null or zero signal at the thermocouple. The servo position is a measure of the sample content. Ordinarily, the wavelength of the radiation source is scanned over a range by a monochromator during the measurement to provide a spectrum measure of the sample. This spectrum is plotted on a chart which can have either a fixed chart with a pen moving along an X-Y axis of the chart or a moving chart having the pen traversing one axis of the chart along a fixed line.
In either type of recording under closed loop servo conditions the wavelength scan speed is controlled according to the speed of response of the instrument dynamics which places the beam attenuator such as a comb into the reference beam path, thus resulting in variable speed of the wavelength scanner according to instrument dynamics. Since the chart or pen speed must be controlled in proportion to the wavelength scan speed, prior art systems have utilized a single variable speed motor with mechanical gearing to both the wavelength drive and chart drive to maintain the ratio. Due to the lash or play in the mechanical gearing and further depending upon the amount of gearing necessary due to physical placement of the wavelength drive and chart drive, precise tracking control is virtually impossible to obtain.